This invention relates to lasers and in particular to gas lasers of the excimer type.
A gas laser is operated by creating a discharge within a gaseous medium within an optical resonator. The discharge causes an over population of upper energy levels within the medium, and subsequent transition to lower energy levels releases light at discrete frequencies within the resonator to maintain a lasing action. In a typical gas laser, a hollow tube confines the discharge between the mirrors of the optical resonator.
Gas lasers often use hollow optical waveguides to excite the laser medium as well as to remove heat while still maintaining a good beam quality. Alternate reflection of the optical rays from opposing surfaces as the optical rays propagate down the waveguide enhances beam quality. Moreover, losses due to reflection rapidly increase at high angles of incidence. Thus the waveguide selectively discriminates against the oscillation of high-divergence optical modes. This further improves beam quality.
However, the hollow waveguide retains heat and uses mainly the conductivity of the walls to remove heat by conduction. This results in a decrease of the extraction efficiency of the laser.
Such devices also operate at gas pressures below one atmosphere and at relatively low excitation levels. At higher gas pressures and high RF power levels, inhomogeneities and instabilities develop in the gas discharge which limits the utility of the laser. The use of metallic electrodes in contact with the laser gas is undesirable with corrosive gas mixtures, especially gas mixtures of the type used with excimer lasers.
In addition, microwave excitation of the gas medium within a hollow waveguide has been employed, in which the microwave excitation has been along the direction of the optical axis of the hollow waveguide and generally occupies the entire volume of the hollow waveguide. When used with a gaseous medium which occupies the entire volume, or a substantial portion of the volume of the hollow waveguide, it is difficult to adjust and maintain the position of the maximum amplitude of the standing E-field wave that is created. This decreases the efficiency that would otherwise be achievable.